The Journal of Immunology, 2007, 179, 6384-6388
Copyright © 2007 by The American Association of Immunologists, Inc.
Cutting Edge: Rac GTPases Sensitize Activated T Cells to Die via Fas1
Madhu Ramaswamy*,
Celine Dumont
,
Anthony C. Cruz*,
Jagan R. Muppidi*,
Timothy S. Gomez
,
Daniel D. Billadeau,
Victor L. J. Tybulewicz and
Richard M. Siegel2,*
* Immunoregulation Unit, Autoimmunity Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892;
Division of Immune Cell Biology, National Institute for Medical Research, Mill Hill, London, United Kingdom; and
Department of Oncology Research, Mayo Clinic, Rochester, MN 55905
 |
Abstract
|
|---|
In activated CD4+ T cells, TCR restimulation triggers apoptosis that depends on interactions between the death receptor Fas and its ligand, FasL. This process, termed restimulation-induced cell death (RICD), is a mechanism of peripheral immune tolerance. TCR signaling sensitizes activated T cells to Fas-mediated apoptosis, but what pathways mediate this process are not known. In this study we identify the Rho GTPases Rac1 and Rac2 as essential components in restimulation-induced cell death. RNA interference-mediated knockdown of Rac GTPases greatly reduced Fas-dependent, TCR-induced apoptosis. The ability of Rac1 to sensitize T cells to Fas-induced apoptosis correlated with Rac-mediated cytoskeletal reorganization, dephosphorylation of the ERM (ezrin/radixin/moesin) family of cytoskeletal linker proteins, and the translocation of Fas to lipid raft microdomains. In primary activated CD4+ T cells, Rac1 and Rac2 were independently required for maximal TCR-induced apoptosis. Activating Rac signaling may be a novel way to sensitize chronically stimulated lymphocytes to Fas-induced apoptosis, an important goal in the treatment of autoimmune diseases.
|
Introduction
|
|---|
Activation of the Fas death receptor by the Fas ligand (FasL)3 is one mechanism of peripheral CD4+ T cell tolerance. Fas-FasL interactions play a role in the apoptosis of activated T cells that are restimulated through the TCR in restimulation-induced cell death (RICD) (1). Physiologically, RICD occurs more often with self rather than foreign Ags, and loss of Fas or FasL leads to systemic autoimmunity in mice and humans even when restricted to T cells (2, 3). T cell contraction after acute stimulation is independent of Fas, but apoptosis after repeated TCR stimulation or ubiquitously expressed Ag is impaired in Fas-deficient T cells (3, 4, 5). TCR restimulation of activated CD4+ T cells results in up-regulation of FasL, which can mediate apoptosis in T cells that express Fas on their surface (1). However, restimulated T cells preferentially undergo apoptosis whereas "bystander"-activated T cells do not (6, 7). It was subsequently found that the TCR delivers a protein synthesis-independent "competency to die" signal that sensitizes activated T cells to FasL-induced apoptosis (6, 7, 8). One mechanism of heightening the efficiency of Fas signaling is the translocation of Fas into lipid raft microdomains (8).
Which TCR signaling pathways mediate the "competency to die" signal is not known. In this study we have identified the Rac small GTPases as critical components of the TCR-induced RICD pathway.
|
Materials and Methods
|
|---|
Cells and transfections
Jurkat and SKW6.4 cells were obtained from American Type Culture Collection. Vav1-deficient Jurkat (J.Vav1) cells were obtained from L. Samelson (National Cancer Institute, (NCI), National Institutes of Health, Bethesda, MD). Transfection of siRNA and plasmids into Jurkat cells or J.Vav1 was done using a BTX ECM 610 electroporator. Mouse CD4+ T cells were isolated from wild-type (WT) or Rac1+/– Rac2–/– mice (129S8) or from strains containing CD2-Cre whose Rac genotypes were Rac1+/+Rac2+/+ (WT), Rac1flox/floxRac2+/+ (Rac1T), or Rac1+/+Rac2–/– (9).
Antibodies and reagents
Abs were obtained from Kamiya (Apo1-1), Alexis Biochemicals (Apo1-3), eBioscience (anti-CD3; clone OKT3), Upstate Biotechnology (Rac1), Cell Signaling Technology (ERK/p-ERK, JNK/p-JNK, p-ERM, where p stands for "phosphorylated"), and Lab Vision (ezrin). Rac1-validated stealth RNA duplex (5'-UGGAGAAUAUAUCCCUACUGUCUUU-3') consisting of a 25-mer synthetic small interfering RNA (siRNA) duplex and Rac2 siRNA were from Invitrogen. The Rac1 and Rac2 short hairpin RNA constructs in pCMS3-H1p-EGFP vectors have been described previously (10). The Rac dominant negative construct was a gift from S. Shaw (NCI) and the FasL construct was from J. Tschopp (University of Lausanne, Lausanne Switzerland). The constitutively active Rac1 mutants were obtained from D. Cantrell (University of Dundee, Dundee, Scotland).
Real-time quantitative RT-PCR analysis
mRNA was reverse transcribed and amplified using the SuperScript One-Step RT-PCR kit with the addition of a 1/50 dilution of ROX reference dye. Primer and probe sets for human FasL, Rac1, Rac2, and β2-microglobulin were obtained from Applied Biosystems. Relative mRNA levels were calculated after normalizing for β2-microglobulin and control threshold cycle (Ct) values.
Quantitation of ezrin/radixin-moesin (ERM) phosphorylation
Jurkat cells were surface stained with anti-CD3 biotin for 30 min on ice and cross-linked with streptavidin at 37°C for the indicated periods of time. p-ERM intracellular staining was performed as described previously (11).
Cell death assays
Cell death assays were performed on Jurkat cells or primary T cells activated for 48 h with Con A or anti-CD3/28 and expanded in IL-2 for a minimum of 4 days. Cells were restimulated with anti-CD3 for 6–8 h, after which cells were stained with annexin/PI for 20 min and analyzed by FACS. Cell death was calculated using the formula [(1-percentage of live treated cells)/(percentage of live untreated cells)], where cells that remained Annexin- and PI-negative were considered as live cells.
Lipid raft analysis
siRNA-transfected Jurkat cells (50 x 106) were treated or left unstimulated with 500 ng/ml anti-CD3 for 90 min and cell membranes were fractionated as described perviously (8).
|
Results and Discussion
|
|---|
TCR-induced sensitization to Fas-induced apoptosis depends on Rac GTPases
The Jurkat T lymphoma cell line is classified as type II in that Fas-induced apoptosis is dependent on mitochondrial amplification. Only cross-linked anti-Fas (Apo1–3 (IgG3) plus anti-IgG3), but not bivalent anti-Fas (Apo-1 (IgG1)) Abs induce efficient apoptosis (8, 12). Stimulation of Jurkat cells with soluble anti-CD3 synergistically induced apoptosis mediated by Apo-1-1, which was similar to our previous results in primary activated human T cells (8) (Fig. 1A). A Rac1 dominant negative construct suppressed TCR sensitization to Fas-induced apoptosis, whereas inhibition of PI3K or NF-
B signaling had no effect (data not shown). Transfection of Jurkat cells with siRNA targeting endogenous Rac1 resulted in almost complete abrogation of TCR-mediated sensitization to Fas (Fig. 1B), whereas cell death induced by cross-linked anti-Fas was unaffected (Fig. 1C). This siRNA oligomer efficiently reduced Rac1 mRNA and protein but not mRNA for Rac2, a highly homologous protein present in immune cells (Fig. 1, D and E). Control and Rac1 siRNA transfection did not affect growth rates and surface levels of Fas and TCR (data not shown). Similar results were obtained with short hairpin RNA constructs targeting different sequences in Rac1 (data not shown). These data indicate that Rac1 is essential for TCR-induced apoptosis through Fas in Jurkat cells.
View larger version (43K):
[in this window]
[in a new window]
|
FIGURE 1. TCR sensitization to Fas-induced apoptosis depends on Rac1. A and B, Jurkat cells transfected with the indicated siRNA were tested for TCR-mediated sensitization to apoptosis induced by bivalent anti-Fas (Apo1-1). C, cells were stimulated with Apo1-3 cross-linked with IgG3. Data are representative of six experiments, ± SEM. D, Specific knockdown of Rac1 demonstrated by real-time quantitative RT-PCR with primers specific for Rac1 or Rac2. E, Immunoblotting for Rac1 in Rac1 or control siRNA-treated cells.
|
|
The cytoskeletal remodeling activity of Rac1 mediates TCR-induced "competency to die" via Fas
In resting T cells, TCR-induced changes in the dynamics of the actin cytoskeleton are important during TCR-induced activation in forming and maintaining the T cell/APC immune synapse and the subsequent proliferation and activation of T cell effector functions. Rac1 mediates this process through the dephosphorylation of ERM family linker proteins, which allows redistribution of ERM-linked surface receptors and morphological changes (11). We found that TCR stimulation of Jurkat cells induces rapid ERM dephosphorylation, which was inhibited by Rac1 siRNA (Fig. 2, A and B). Rac GTPases are activated upon TCR stimulation by the Vav1 GTP/GDP exchange factor. However, Vav-dependent events such as calcium flux, activation of JNK and ERK kinases, and NF-B activity were not affected by Rac1 siRNA (data not shown), indicating that Rac1 controls a more restricted set of signaling pathways than Vav1.
View larger version (37K):
[in this window]
[in a new window]
|
FIGURE 2. Fas sensitization depends on Rac-1-mediated cytoskeletal modulation. A, Intracellular p-ERM levels in the Jurkat cells treated with Rac1 or control siRNA and then stimulated with anti-CD3. Gray histogram is isotype control, thick line denotes untreated cells, and thin line represents cells treated with anti-CD3 for 5 min. B, Time course of p-ERM dephosphorylation generated from the geometric mean of intracellular p-ERM staining. Data are representative of four experiments. C, TCR-mediated sensitization to bivalent anti-Fas was tested in J.Vav1 Vav-deficient Jurkat cells cotransfected with a GFP expression vector and indicated dominant active mutants of Rac1. Specific cell death was assessed by annexin/PI staining of the GFP-positive population. All data are representative of three independent experiments and error bars are ± SEM (*, p < 0.05; ns, nonsignificant per paired t test).
|
|
To determine which functions of Rac1 are sufficient to mediate TCR-induced Fas sensitization, we studied J.Vav1 cells, which are unable to activate Rac1 after TCR stimulation (13). These cells are completely resistant to TCR-induced sensitization to Apo-1-1, but still undergo apoptosis in response to cross-linked anti-Fas (data not shown). Transfection of RacQ61L, a constitutively active mutant of Rac1, restored the ability of TCR stimulation to sensitize to Apo-1-1-induced apoptosis in J.Vav1 cells (Fig. 2C). Transfection with RacQ61L/Y40C, a mutant that is deficient in the activation of MAP kinases but not in the induction of cytoskeletal changes (14), also rescued "competency to die." However, cells transfected with the RacQ61L/F37A mutant that is defective in modifying the cytoskeleton exhibited reduced and statistically insignificant TCR-induced sensitization to Fas (Fig. 2C). These data suggest that Rac1 effector functions that activate cytoskeletal remodeling are necessary to sensitize cells to Fas-induced apoptosis. However, the fact that TCR signaling is still necessary to sensitize cells to Fas-induced apoptosis, even in the presence of a dominant active Rac1, indicates that other Vav1-independent signals are also required.
Rac1 is required for TCR-induced apoptosis mediated by endogenous FasL and blocks TCR-induced lipid raft translocation of Fas
TCR-induced apoptosis depends on both the sensitization of Fas signaling and de novo FasL synthesis. Unlike sensitization signals induced by weak TCR stimuli such as soluble anti-CD3, cross-linked anti-CD3 leads to the induction of FasL, which can then trigger apoptosis. As shown in Fig. 3A, Rac1 siRNA potently blocked the ability of Jurkat cells to undergo apoptosis in response to cross-linked anti-CD3. Surprisingly, Rac1 siRNA also reduced the transcriptional up-regulation of FasL via anti-CD3 (Fig. 3B). To determine whether this reduction in FasL production was biologically significant, we tested the ability of Rac1 siRNA-treated Jurkat cells to kill the Fas-sensitive B cell line SKW 6.4 after stimulation with bead-bound anti-CD3. Rac1 siRNA significantly reduced target cell apoptosis at two different effector-target ratios (Fig. 3C). To determine whether FasL production is limiting or whether Rac1 delivers an independent Fas sensitization signal, we labeled control siRNA-treated Jurkat cells with CFSE and mixed them at a 1:1 ratio with unlabeled Rac1 siRNA-treated cells. These cell mixtures were then stimulated with anti-CD3 and assessed for apoptosis. As shown in Fig. 3D, the addition of Rac1-sufficient cells could not rescue the apoptosis defect in the Rac1 siRNA-treated cells.
View larger version (56K):
[in this window]
[in a new window]
|
FIGURE 3. Rac1 enhances FasL production, sensitivity to FasL-mediated apoptosis, and TCR-induced translocation of Fas to lipid rafts. A, Jurkat cells pretreated with the indicated siRNA were stimulated with anti-CD3 beads and assessed for apoptosis using annexin/PI staining. B, FasL was measured with real-time quantitative RT-PCR from siRNA-treated Jurkat cells stimulated with anti-CD3 for the indicated times. C, Control and Rac1 siRNA-treated Jurkat cells were labeled with CFSE and mixed with unlabeled SKW 6.4 target cells at the indicated ratios. The mixed population was stimulated with anti-CD3 beads and assessed for apoptosis of SKW 6.4 cells using annexin/PI staining on the CFSE-negative population. D, Cultures of control (CFSE-labeled) and Rac1 siRNA-transfected Jurkat cells (unlabeled) were stimulated with anti-CD3 beads either in mixed (Rac1 plus control, hatched bar) or separate (Control, filled bar; Rac1, gray bar) cultures and assessed for apoptosis. E, FasL was transfected into control and Rac1 siRNA Jurkat cells. Cells were subsequently stimulated with soluble anti-CD3 and specific cell death was assessed using annexin/PI. F, Fas translocation upon TCR stimulation is blocked in Jurkat cells transfected with Rac1 siRNA (lower panel). Lck is shown as a control for lipid raft localization. All data represent at least two individual experiments and error bars are ± SEM. p values are from two-tailed paired t test: *, p < 0.05; **, p < 0.005; ***, p < 0.0005.
|
|
We also tested whether the transfection of FasL could restore apoptosis in Rac1-depleted Jurkat cells. As previously reported (8), transfection of FasL did not induce apoptosis alone, but as little as 10 ng/ml soluble anti-CD3 sensitized FasL-transfected cells to die. Rac1 depletion via siRNA significantly reduced the ability of TCR stimulation to sensitize FasL transfected cells to die (Fig. 3E). These results indicate that Rac1 is required in a cell-autonomous fashion to transduce the TCR "competency to die" signal independently of its effect on FasL synthesis. Importantly, these experiments also show that Rac1 is involved in sensitizing T cells to apoptosis induced by endogenously synthesized FasL.
We have previously shown that TCR-induced sensitization to Fas requires Fas translocation to the detergent-insoluble lipid raft fraction of the membrane (8). We therefore asked whether Rac1 siRNA suppresses lipid raft translocation of Fas. In control Jurkat cells, TCR stimulation increased the fraction of Fas in lipid rafts 5-fold (Fig. 3F, top panel, from 6.68 to 34.32%). Rac1 siRNA-transfected Jurkat cells exhibited dramatic inhibition of Fas translocation to lipid rafts (Fig. 3F, lower panel, from 7.28 to 0.39%), indicating that Rac1 is responsible for this key step in the acquisition of sensitivity to die via Fas.
To investigate the role of Rac GTPases in the apoptosis of primary T cells, we introduced Rac1 siRNA into activated human CD4+ T cells via nucleofection. As seen in Fig. 4, A and B, Rac1 siRNA suppressed TCR-mediated sensitization to bivalent anti Fas-induced apoptosis but did not affect the responses to cross-linked anti-Fas (data not shown). Rac1 siRNA was also efficient in reducing endogenous Rac1 protein levels in these cells (Fig. 4B, Inset). When Rac2 siRNA or a mixture of Rac1 and Rac2 siRNA was used, we found significant suppression of anti-CD3-induced death (Fig. 4C). Thus both Rac1 and Rac2, with the predominant contribution of Rac2, are likely involved in TCR-induced apoptosis of primary T cells. CD44, a surface glycoprotein that is highly expressed on activated T cells, is a hyaluronan receptor and participates in cell adhesion and motility through interaction with and modification of the extracellular matrix (15). CD44 is known to induce cytoskeletal changes through the activation of Rac1 (16). Strikingly, the treatment of human activated T cells with anti-CD44 sensitized cells to Fas-induced apoptosis as efficiently as anti-CD3 (Fig. 4D).
View larger version (44K):
[in this window]
[in a new window]
|
FIGURE 4. Requirement for Rac GTPases in RICD of primary T cells and the ability of CD44 to substitute for the TCR. A and B, Activated human CD4+ T cells were transfected with control (A) or Rac1 (B) siRNA for 96 h and sensitization to Apo-1-1 by anti-CD3 was tested (dashed line, medium, solid line, TCR). Specific cell death was measured 8 h after restimulation. Immunoblots of endogenous Rac1 is indicated (B, Inset). C, Sensitivity of Rac1, Rac2, or Rac1 and Rac2 siRNA-transfected CD4+ T cells to bead-bound anti-CD3. D, Anti-CD44 sensitization to Fas-induced apoptosis. Data are representative of at least three independent experiments (error bars are SEM). E, CD4+ T cells from Rac1T or Rac2–/– mice and isogenic Rac-sufficient controls were restimulated on plate-bound anti-CD3 for 9 h and the specific cell death was determined (n = 6). F, RICD in activated CD4+ T cells derived from compound Rac1-deficient mice (Rac1–/+Rac2–/–) vs wild-type T cells on a matched genetic background. Significant differences between WT and the knockout mice at various CD3 concentrations are indicated (n = 2). p values are from two-tailed paired t test: *, p < 0.05; **, p < 0.005; ***, p < 0.0005.
|
|
To extend these studies to cells genetically deficient in Rac GTPases, we examined activated mouse T cells that have been engineered to lack Rac1 or Rac2 (C. Dumont, M. Walmsley, and V. L. Tybulewicz, unpublished data). Rac2 knockout (Rac2–/–) mice are viable and have normal T cell development (17). T cell specific Rac1 knockout mice (Rac1T) were generated by crossing previously described mice homozygous for a loxP flanked Rac1 allele (Rac1flox/flox) with CD2-Cre mice that delete Rac1 early in T cell development. As seen in Fig. 4E, the absence of Rac1 as well as Rac2 decreases the efficiency of TCR-induced apoptosis in activated T cells from both Rac1T and Rac2–/– deficient mice. To study the redundancy of Rac1 and Rac2 in RICD, CD4+ T cells from compound Rac-deficient mice in which one allele of Rac1 was left intact (Rac1–/+ Rac2–/–) were used. Rac1–/–Rac2–/– deficient T cells could not be obtained because of severe T cell developmental defects in Rac1T Rac2–/– mice (C. Dumont and V. L. J. Tybulewicz, unpublished data). Rac1–/+Rac2–/– CD4+ T cells had greater defects in RICD than Rac1T or Rac2–/– T cells when compared with the WT mice of the same genetic background (Fig. 4F). T cells from all of these Rac mutants underwent normal apoptosis in response to oligomerized FasL (FasL LZ-FLAG), a strong inducer of Fas apoptosis (WT, 72.5%; Rac1T, 71.3%; Rac2–/–, 75.7%; Rac1–/+Rac2–/–, 64%, p = NS), indicating that Fas signaling itself is not dependent on Rac GTPases.
In this study, we have shown that Rac small GTPases are essential for sensitization of activated T cells to undergo Fas-mediated apoptosis. Lipid raft translocation of Fas, which is necessary for Fas sensitization, is also reversed by Rac1 siRNA. Rac1 promotes microfilament remodeling and repositioning of the centrosome toward the area of contact with target cells (18). Actin reorganization is dependent on the F37 residue of Rac1, which was required for Fas sensitization. Although Fas sensitization in Jurkat cells relies mainly on Rac1, both Rac1 and Rac2 likely play a role in TCR-induced apoptosis of primary T cells. Primary T cells lacking one Rac1 allele and both Rac2 alleles were more impaired in RICD than Rac2-deficient cells alone, indicating likely cooperation between these two Rac isoforms in mediating RICD. Although Rac2-deficient cells have been reported to have activation defects (19), under the conditions used here proliferation, viability, and activation marker expression were not impaired in either Rac1- or Rac2-deficient T cells (data not shown).
Our results suggest that the modulation of Rac1 though the TCR or other receptors may alter the threshold for Fas-mediated elimination of chronically stimulated T cells. We have not observed autoantibody production or autoimmune pathology in younger Rac-deficient mice, but this will need to be studied further. Signaling through the TCR or another surface molecule, CD44, which also activates Rac1, enhanced Fas-induced apoptosis. T cells from CD44-deficient mice are resistant to RICD and also had increased pathology in an autoimmune hepatitis model regulated by Fas-mediated elimination of activated T cells (20). Activation of Rac1 may be a novel way to enhance the sensitivity of autoreactive T cells to apoptosis, a strategy that could be useful in the therapy of autoimmune diseases.
|
Acknowledgments
|
|---|
We thank Doreen Cantrell (Wellcome Trust Biocenter, University of Dundee, Dundee, Scotland) and Steve Shaw (National Cancer Institute, National Institutes of Health, Bethesda, MD) for reagents and advice, and Mike Lenardo for critical reading of the manuscript.
|
Disclosures
|
|---|
The authors have no financial conflict of interest.
|
Footnotes
|
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This research was supported in part by the Intramural Research Program of National Institute of Arthritis and Musculoskeletal and Skin Diseases. 
2 To whom correspondence should be addressed: Dr. Richard M. Siegel, Building 10, Room 9N238, Bethesda, MD 20892. E-mail address: rsiegel{at}nih.gov
3 Abbreviations used in this paper: FasL, Fas ligand; ERM, ezrin/radixin/moesin; J.Vav1, Vav1-deficient Jurkat; RICD, restimulation-induced cell death; siRNA, small interfering RNA; WT, wild type.
Received for publication June 27, 2007.
Accepted for publication September 26, 2007.
|
References
|
|---|
- Siegel, R. M., F. K. Chan, H. J. Chun, M. J. Lenardo. 2000. The multifaceted role of Fas signaling in immune cell homeostasis and autoimmunity. Nat. Immunol. 1: 469-474. [Medline]
- Holzelova, E., C. Vonarbourg, M. C. Stolzenberg, P. D. Arkwright, F. Selz, A. M. Prieur, S. Blanche, J. Bartunkova, E. Vilmer, A. Fischer, et al 2004. Autoimmune lymphoproliferative syndrome with somatic Fas mutations. N. Engl. J. Med. 351: 1409-1418. [Abstract/Free Full Text]
- Stranges, P. B., J. Watson, C. J. Cooper, C. M. Choisy-Rossi, A. C. Stonebraker, R. A. Beighton, H. Hartig, J. P. Sundberg, S. Servick, G. Kaufmann, et al 2007. Elimination of antigen-presenting cells and autoreactive T cells by Fas contributes to prevention of autoimmunity. Immunity 26: 629-641. [Medline]
- Ettinger, R., D. J. Panka, J. K. Wang, B. Z. Stanger, S. T. Ju, A. Marshak-Rothstein. 1995. Fas ligand-mediated cytotoxicity is directly responsible for apoptosis of normal CD4+ T cells responding to a bacterial superantigen. J. Immunol. 154: 4302-4308. [Abstract]
- Fisher, G. H., F. J. Rosenberg, S. E. Straus, J. K. Dale, L. A. Middleton, A. Y. Lin, W. Strober, M. J. Lenardo, J. M. Puck. 1995. Dominant interfering Fas gene mutations impair apoptosis in a human autoimmune lymphoproliferative syndrome. Cell 81: 935-946. [Medline]
- Hornung, F., L. Zheng, M. J. Lenardo. 1997. Maintenance of clonotype specificity in CD95/Apo-1/Fas-mediated apoptosis of mature T lymphocytes. J. Immunol. 159: 3816-3822. [Abstract]
- Combadiere, B., C. Reis e Sousa, C. Trageser, L. X. Zheng, C. R. Kim, M. J. Lenardo. 1998. Differential TCR signaling regulates apoptosis and immunopathology during antigen responses in vivo. Immunity 9: 305-313. [Medline]
- Muppidi, J. R., R. M. Siegel. 2004. Ligand-independent redistribution of Fas (CD95) into lipid rafts mediates clonotypic T cell death. Nat. Immunol. 5: 182-189. [Medline]
- Walmsley, M. J., S. K. Ooi, L. F. Reynolds, S. H. Smith, S. Ruf, A. Mathiot, L. Vanes, D. A. Williams, M. P. Cancro, V. L. Tybulewicz. 2003. Critical roles for Rac1 and Rac2 GTPases in B cell development and signaling. Science 302: 459-462. [Abstract/Free Full Text]
- Gomez, T. S., M. J. Hamann, S. McCarney, D. N. Savoy, C. M. Lubking, M. P. Heldebrant, C. M. Labno, D. J. McKean, M. A. McNiven, J. K. Burkhardt, D. D. Billadeau. 2005. Dynamin 2 regulates T cell activation by controlling actin polymerization at the immunological synapse. Nat. Immunol. 6: 261-270. [Medline]
- Faure, S., L. I. Salazar-Fontana, M. Semichon, V. L. Tybulewicz, G. Bismuth, A. Trautmann, R. N. Germain, J. Delon. 2004. ERM proteins regulate cytoskeleton relaxation promoting T cell-APC conjugation. Nat. Immunol. 5: 272-279. [Medline]
- Scaffidi, C., I. Schmitz, J. Zha, S. J. Korsmeyer, P. H. Krammer, M. E. Peter. 1999. Differential modulation of apoptosis sensitivity in CD95 type I and type II cells. J. Biol. Chem. 274: 22532-22538. [Abstract/Free Full Text]
- Cao, Y., E. M. Janssen, A. W. Duncan, A. Altman, D. D. Billadeau, R. T. Abraham. 2002. Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line. EMBO J. 21: 4809-4819. [Medline]
- Lamarche, N., N. Tapon, L. Stowers, P. D. Burbelo, P. Aspenstrom, T. Bridges, J. Chant, A. Hall. 1996. Rac and Cdc42 induce actin polymerization and G1 cell cycle progression independently of p65PAK and the JNK/SAPK MAP kinase cascade. Cell 87: 519-529. [Medline]
- Turley, E. A., P. W. Noble, L. Y. Bourguignon. 2002. Signaling properties of hyaluronan receptors. J. Biol. Chem. 277: 4589-4592. [Free Full Text]
- Oliferenko, S., I. Kaverina, J. V. Small, L. A. Huber. 2000. Hyaluronic acid (HA) binding to CD44 activates Rac1 and induces lamellipodia outgrowth. J. Cell Bio. 148: 1159-1164. [Abstract/Free Full Text]
- Roberts, A. W., C. Kim, L. Zhen, J. B. Lowe, R. Kapur, B. Petryniak, A. Spaetti, J. D. Pollock, J. B. Borneo, G. B. Bradford, et al 1999. Deficiency of the hematopoietic cell-specific Rho family GTPase Rac2 is characterized by abnormalities in neutrophil function and host defense. Immunity 10: 183-196. [Medline]
- Gomez, T. S., K. Kumar, R. B. Medeiros, Y. Shimizu, P. J. Leibson, D. D. Billadeau. 2007. Formins regulate the actin-related protein 2/3 complex-independent polarization of the centrosome to the immunological synapse. Immunity 26: 177-190. [Medline]
- Yu, H., D. Leitenberg, B. Li, R. A. Flavell. 2001. Deficiency of small GTPase Rac2 affects T cell activation. J. Exp. Med. 194: 915-926. [Abstract/Free Full Text]
- Chen, D., R. J. McKallip, A. Zeytun, Y. Do, C. Lombard, J. L. Robertson, T. W. Mak, P. S. Nagarkatti, M. Nagarkatti. 2001. CD44-deficient mice exhibit enhanced hepatitis after concanavalin A injection: evidence for involvement of CD44 in activation-induced cell death. J. Immunol. 166: 5889-5897. [Abstract/Free Full Text]
This article has been cited by other articles:
|
|
|
|
 
C. Petrovas, B. Chaon, D. R. Ambrozak, D. A. Price, J. J. Melenhorst, B. J. Hill, C. Geldmacher, J. P. Casazza, P. K. Chattopadhyay, M. Roederer, et al.
Differential Association of Programmed Death-1 and CD57 with Ex Vivo Survival of CD8+ T Cells in HIV Infection
J. Immunol.,
July 15, 2009;
183(2):
1120 - 1132.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
W. Ruan, C. T. Lee, and J. Desbarats
A Novel Juxtamembrane Domain in Tumor Necrosis Factor Receptor Superfamily Molecules Activates Rac1 and Controls Neurite Growth
Mol. Biol. Cell,
August 1, 2008;
19(8):
3192 - 3202.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
